1. Field of the Invention
The present invention relates to a communication unit having a non-destruction arbitration function for use in a data communication system implementing a CSMA/CD (carrier sense multiple access/collision detection) method for conducting serial communication by implementing pulse-width modulation or a similar technique on digital data.
2. Description of the Related Art
In the past, a CSIA/CD data communication system having a non-destructive arbitration function has been known. This system has the following functions: (1) starting transmission of transmission data when no transmission data from another communication unit is present on a transmission line, (2) monitoring collision of the transmission data being transmitted with transmission data from other communication units on the transmission line during transmission of the transmission data, (3) causing the communication unit transmitting the transmission data having higher priority to continue to transmit the transmission data when the transmission data collide and (4) causing the communication unit transmitting the transmission data having lower priority to halt the transmission of the transmission data. This kind of system is described in U.S. Pat. No. 4,745,596. Also, the J1850 Standard, which is an international standard is defined according to this system.
This communication unit confirms whether the transmission data which it outputted is flowing on the transmission line per one bit of the transmission data and if the same data with the transmission data which it has transmitted is flowing on the transmission data, it continues to transmit the transmission data. If the same data with the transmission data which it transmitted is not flowing on the transmission line, it stops transmitting the transmission data.
Accordingly, when each communication unit composing a data communication system operates as described above and when three communication units A, B and C as shown, for example, in FIG. 12, start to transmit transmission data simultaneously at time t1, each of the communication units continues to transmit the transmission data when the transmission data from each of the communication units A, B and C agree and the same transmission data with the transmission data which are transmitted from each of the communication units A, B and C is flowing on the transmission line L. Then, when the transmission data from each of the communication units A, B and C begin to disagree and when transmission data having higher priority among them (higher level has a priority in FIG. 12) starts to flow on the transmission line L, the communication units B and A which have transmitted transmission data having less priority halt transmitting the transmission data one by one and in the end, only the communication unit C continues to transmit the transmission data.
In the CSMA/CD data communication system having such a non-destructive arbitration function, the transmission data may be outputted at any time when another communication unit is not outputting transmission data on the transmission line. However, since the communication unit transmits/receives transmission data through the intermediary of a known transmission line driver/receiver (hereinafter referred to simply as a D/R), a delay time is generated when the communication unit receives the transmission data on the transmission line due to the transmission delay of the D/R or the floating capacity of the transmission line.
Due to this fact, as shown in FIG. 13 for example, even if one communication unit starts transmission of transmission data from a transmission terminal TX1, the communication unit and other communication units receive the transmission data with a delay of delay time .DELTA.d (TX1' shown in the Figure) and another communication unit may start transmitting transmission data from a transmission terminal TX2 during that delay time .DELTA.d. In this case, since each of the communication units receives the transmission data from the transmission terminal TX2 in a delay of the delay time .DELTA.d (TX2' shown in the Figure), a signal in which the transmission data TX1' and TX2' from each of the communication units are combined is inputted to receiving terminals RX1 and RX2 of each of the communication units in the end. The input data has become data which is different from the transmission data transmitted by each of the communication units (data whose shaded portion has been changed from Low level to High level), and normal data communication may not be executed.
On the other hand, in this type of data communication system, since each communication unit operates in accordance with own the system clocks, system clock may differ between each communication unit and periods of transmission data to be transmitted may not agree. In this case, as shown in FIG. 14 for example, even if two communication units start transmitting transmission data from transmission terminals TX1 and TX2 with the same timing, a signal which is different from the transmission data transmitted by each of the communication units appears on the transmission line L due to the deviation of the periods of each transmission data and each of the communication units may end up halting the transmission of the transmission data altogether.